The invention relates to a method of providing an epitaxial indium phosphide layer on a substrate surface by bringing a gas mixture comprising a metalorganic indium compound and phosphine into contact with the heated substrate surface.
Such indium phosphide layers (InP layers) are used in optoelectronic components such as solid-state lasers, light-emitting diodes (LEDs), photodetectors and electronic components such as high-frequency field effect transistors. Processes for applying such layers from the vapour phase, in which metalorganic indium compounds are used are referred to in literature as MOCVD processes (MOCVD=metalorganic chemical vapour deposition) or OMCVD (OMCVD=organometallic chemical vapour deposition). If epitaxial layers are provided by means of such a method, this is commonly referred to as MOVPE-processes (MOVPE=metalorganic vapour phase epitaxy). In order to apply an InP layer to a substrate by means of a MOVPE process, a carrier gas (for example H.sub.2) is conducted over a suitable metalorganic indium compound such as, for example, trimethyl indium ((CH.sub.3).sub.3 In), in which the (CH.sub.3).sub.3 In vapour is led over the heated substrate together with phosphine (PH.sub.3), the following reaction taking place: EQU (CH.sub.3).sub.3 In+PH.sub.3 .fwdarw.InP+3CH.sub.4
Triethyl indium may alternatively be used instead of trimethyl indium. However, when the said indium reagents (precursors) are used, undesirable side reactions occur between the indium compounds and phosphine. These side reactions are caused by the Lewis acid character of the indium compounds on the one hand and the Lewis base character of phosphine on the other hand.
A Lewis acid is an element or compound having an electron deficiency, as a consequence hereof it is an electron acceptor such as the trialkyl compounds of elements in group III of the periodic system to which In belongs. A Lewis base is an element or compound having an electron excess, as a consequence of which it is an electron donor such as the trialkyl compounds and hydrides of elements in group IV to which P belongs. Due to the Lewis acid and the Lewis base character, respectively, of the reagents used an adduct is formed: EQU (CH.sub.3).sub.3 In+PH.sub.3 .fwdarw.(CH.sub.3).sub.3 In--PH.sub.3
The adduct formed decomposes spontaneously while splitting off methane molecules, resulting in a polymer compound being formed: EQU --InCH.sub.3 PH--.sub.n.
This polymer compound is non-volatile and is deposited from the vapour phase. The undesirable polymer formation takes place at room temperature, before the reagents can reach the heated substrate. This polymer formation causes deletion of reactants in the vapour phase, which leads to an irregular and low growth rate of the InP layer. Due to the deposition of the polymer carbon is incorporated in the growing layer, which adversely affects the desired luminescing and/or electronic properties of the InP layer. Moreover, the presence of the amorphous polymeric material on the growing InP surface disturbs the epitaxial crystal growth.
In European Patent Specification EP No. 52979, a method is described by means of which the above-said undesirable polymer formation is precluded. In accordance with the said method, firstly, an adduct of an indium compound, for example (CH.sub.3).sub.3 In, and a trialkyl phosphine, for example P(C.sub.2 H.sub.5).sub.3 is prepared. The adduct (CH.sub.3).sub.3 In--P(C.sub.2 H.sub.5).sub.3 is formed in a separate reaction space at a low temperature. Unlike the adducts of PH.sub.3, an adduct thus formed is chemically stable and does not cause polymers to be formed. Subsequently, the adduct formed is mixed in a second space, an additional quantity of phosphine being used to prepare an InP layer on a substrate. To obtain a phosphor-containing layer, phosphine must be used because of the thermal stability of the triethyl phosphine group.
A disadvantage of the known method is the low vapour pressure of the adducts comprising trialkyl phosphines, which leads to a limitation of the growth rate of the InP layer. Moreover, the already complicated MOVPE process is rendered even more complicated.